Developments in Ceramic Materials Research

Coloured ZrSiO4 Ceramic Pigments 271
relevant role in promoting the zircon structure, the promotion being more efficient for small
monovalent cations and less efficient for large monovalent or divalent cations.
Monros [34] and Lusar [35] report, in the case of V-doped pigments, added with NaF, a
promotion of the zircon structure at lower temperatures with respect to the absence of
mineralizers and a contraction of the unit cell volume, consistently with results obtained by
our group [24]. These authors interpret the contraction of the unit cell volume as due to the
partial substitution of O 2- in the lattice by smaller F - ions.
In this work the role played by several mineralizers, directly added to the sol-gel mixture
was investigated. The alkaline cations sequence, both as fluorides and also as chlorides, was
investigated to highlight the possible role of either (or both) cations and anions. All
mineralizers were added at the constant molar ratio of 0.26 which, in the case of NaF, was
found to be the amount best promoting the zircon structure (at 800°C) and also yielding the
cell volume closer to the one of pure zircon [24]. To the authors best knowledge, no such
systematic investigation is present in the literature.
The evolution of the zircon percentage with the temperature of calcination for the present
samples obtained at constant vanadium content (V/Zr = 0.1) and in the presence of different
mineralizers is shown in Figure 7.
Figure 7. Zircon-phase mass per cent for V 0.10 doped pigments calcined at 600, 800, 1000°C as for
different mineralizers (0.26 min/Zr molar ratio).
As a general trend the promotion of the zircon phase increases with increasing the
temperature of calcination, both in the absence and in the presence of mineralizers. In the
absence of mineralizers, although the fraction of zircon phase may be relevant, the colour
never becomes blue. At 600°C the zircon phase is formed only in the presence of Li salts, to
larger extents in the case of LiF which promotes the formation of the zircon phase up to 90%.
At any temperature Li salts promote the zircon phase better than any other mineralizer; in the
presence of LiF the zircon percentage is 100% both at 800 and 1000°C. The colour of the
pigments promoted by Li salts is in any case light blue (see in the following Figures 10 and
11).